A new report from the advocacy group Mercury Policy Project finds tuna served as school lunches in some states may contain levels of mercury that the organization deems toxic.

Mercury is a naturally occurring neurotoxin in the environment that can be released into the air through industrial pollution. It builds up in water and streams and turns into methylmercury, a compound which is then absorbed by fish as they feed.

The report, called "Tuna Surprise" is the first to test canned tuna sold to schools, according to its authors.

"Most children are already consuming only modest amounts of tuna and are not at significant risk," Michael Bender, director of the Mercury Policy Project, said in a statement. "So the focus really needs to be on kids who eat tuna often, to limit their mercury exposure by offering them lower-mercury seafood or other nutritious alternatives."

According to the Environmental Protection Agency (EPA), nearly all fish and shellfish contain trace levels of mercury, and some can contain higher levels than others. Methylmercury has been tied to harmful effects on unborn babies' and young children's developing nervous systems, according to the EPA.

In 2004, the U.S. Food and Drug Administration recommended that pregnant women, nursing moms and young kids should avoid certain types of fish high in mercury including shark, swordfish and king mackerel. The agency also said commonly eaten canned albacore "white" tuna has more mercury than canned light tuna, and recommends eating only 6 ounces (or one meal) of white tuna per week.

The watchdog group the Center for Science in the Public Interest - one of the study's sponsors - claims that canned tuna is "by far the largest source of methylmercury in the U.S. diet" and accounts for one-third of American's exposure to this toxic compound of mercury. What's more, the group says studies since the government's recommendation have found adverse effects of methylmercury may occur at lower levels than previously through.

For the new study, researchers obtained 59 tuna samples (35 large cans, 24 foil pouches) from 11 states in the market sector that sells tuna to schools.

Overall, the average mercury level was 0.118 micrograms per gram of tuna, slightly lower than the FDA-reported average of 0.128 micrograms per gram. However, 11 samples of albacore tuna tested had an average of 0.560 parts per million (ppm) of methylmercury, much higher than the FDA-reported average of 0.350 ppm.

Mercury levels very considerably within from sample to sample, with light tuna samples ranging from 0.020 to 0.640 ppm, and for white tuna, 0.190 to 1.270 ppm.

Nine of the samples were from the U.S. and had the lowest mercury level by country-of-origin. The other 50 samples were imports, with tuna from Ecuador having "by far the highest average level," with 0.254 ppm of methylmercury.

The report's authors recommend that children should avoid albacore tuna entirely given it contains roughly three times as much mercury, and said smaller children weighing less than 55 pounds should eat light tuna no more than once a month. Most children should eat light tuna only twice a month, the group said.

They warned kids should not be allowed to eat tuna every day and parents should identify kids who "love tuna" and eat it often to try and limit them to only two meals per month. The authors call on the USDA to phase out subsidies for tuna in the school lunch program.

"There is no sound reason why taxpayer dollars should be used to subsidize any part of this risk," the group wrote.

A spokesperson for the USDA told CBSNews.com that it would not be supplying any tuna for this year's school year anyway because it did not have any domestic suppliers for the 2012-2013 year.

The full report from the Mercury Policy Project can be accessed here.

Gavin Gibbons of the industry group the National Fisheries Institute, disputed the study to USA Today.

"To suggest we're eating too much is almost comical," he said. Scaring kids away from tuna "at a point in their life when they're developing their nutrition habits and their palates" is damaging.

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